The present invention relates generally to a height or level control system for an automotive vehicle, which automatically controls vehicular profile depending upon the load on the vehicle and/or vehicular driving conditions. More specifically, the invention relates to an automatic height control system for an automotive vehicle which includes a fail-safe system ensuring maintenance of an acceptable vehicle profile.
Automatic level regulators or vehicular height control systems have been proposed in various constructions and systems. Some of such systems are designed to control not only the vehicular profile but also vehicular height, i.e. road clearance, depending upon vehicle driving conditions and road conditions. One typical construction of the vehicle height control system employs a pressure control for a working fluid in the vehicular suspension system. The fluid pressure is introduced and discharged from a height-control pressure chamber installed in the vehicular suspension system so as to control the vehicle height and/or vehicular profile. However, the damping characteristics of the suspension system tend to vary with the fluid pressure in the height-control pressure chamber.
Specifically, when the fluid pressure in the high-control pressure chamber increases, the suspension system becomes harder. On the other hand, by reducing the fluid pressure in the pressure chamber, the suspension system naturally becomes softer. This sometimes affects drivability and/or riding comfort of the vehicle. Therefore, in order to compensate for this, a vehicle height-control system cooperative with a suspension control system has been proposed to adjust the damping characteristics of the suspension system, in the prior art. In this conventional vehicle height control system cooperating with the suspension control system, primary and auxiliary pressure chambers respectively control the vehicle height and/or profile and the damping characteristics of the suspension system. A pressure control valve is disposed between the primary and auxiliary pressure chambers and within a communication passage. In this case, when the suspension system is to be stiffened, the pressure control valve is closed to block fluid communication between the primary and the auxiliary fluid chambers. In this condition, if height adjustment is performed by adjusting the fluid pressure in the primary chamber, the pressure in the primary and auxiliary chambers differ to a significant exert. If height control is performed to lower the vehicle height under these conditions, the vehicle height would be lowered excessively, causing a temporary, unsatable vehicle driving state.